FIG. 1 is a network structure of an LTE (Long Term Evolution) system, the related art mobile communication system. For the LTE system, which has evolved from the existing UMTS system, basic standardizations are ongoing in the 3GPP.
An LTE network can be divided into an E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) and a CN (Core Network). The E-UTRAN includes a terminal (or UE (User Equipment)), a base station (eNB (Evolved NodeB), and an access gateway (aGW). The access gateway may be divided into a part that handles processing of user traffic and a part that handles control traffic. In this case, the access gateway part that processes the user traffic and the access gateway part that processes the control traffic may communicate with a new interface. One or more cells may exist in a single eNB. An interface may be used for transmitting user traffic or control traffic between eNBs. The CN may include the access gateway and a node or the like for user registration of the UE. An interface for discriminating the E-UTRAN and the CN may be used.
FIG. 2 shows an exemplary structure of a control plane of a radio interface protocol between the UE and the E-UTRAN based on the 3GPP radio access network standards. FIG. 3 shows an exemplary structure of a user plane of the radio interface protocol between the UE and the E-UTRAN based on the 3GPP radio access network standards.
The structure of the radio interface protocol between the UE and the E-UTRAN will now be described with reference to FIGS. 2 and 3.
The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data and a control plane (C-plane) for transmitting control signals. The protocol layers in FIGS. 2 and 3 can be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in the communication system. The radio protocol layers exist as pairs between the UE and the E-UTRAN and handle a data transmission in a radio interface.
The layers of the radio protocol control plane of FIG. 2 and those of the radio protocol user plane of FIG. 3 will be described as follows.
The physical layer, the first layer, provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to an upper layer called a medium access control (MAC) layer via a transport channel. Data is transferred between the MAC layer and the physical layer via the transport channel. The transport channel is divided into a dedicated transport channel and a common channel according to whether or not a channel is shared. Between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transmitted via the physical channel.
The second layer includes various layers. First, a medium access control (MAC) layer performs mapping various logical channels to various transport channels and performs logical channel multiplexing by mapping several logical channels to a single transport channel. The MAC layer is connected an upper layer called a radio link control (RLC) layer by a logical channel. The logical channel is divided into a control channel that transmits information of the control plane and a traffic channel that transmits information of the user plane according to a type of transmitted information.
An RLC (Radio Resource Control) layer, the second layer, segments and/or concatenates data received from an upper layer to adjust the data size so as for a lower layer to suitably transmit the data to a radio interface. In addition, in order to guarantee various QoSs (Quality of Services) required by each radio bearer RB, the RLC layer provides three operational modes: a TM (Transparent Mode); a UM (Unacknowledged Mode); and an AM (Acknowledged Mode). In particular, the RLC layer (referred to as an ‘AM RLC layer’, hereinafter) operating in the AM performs a re-transmission function through an automatic repeat and request (ARQ) function for a reliable data transmission.
A packet data convergence protocol (PDCP) layer of the second layer performs a function called header compression that reduces the size of a header of an IP packet, which is relatively large and includes unnecessary control information, in order to effectively transmit the IP packet such as an IPv4 or IPv6 in a radio interface having a narrow bandwidth. The header compression increases a transmission efficiency between radio interfaces by allowing the head part of the data to transmit only the essential information.
The RRC layer located at the lowermost portion of the third layer is defined only in the control plane, and controls a logical channel, a transport channel and a physical channel in relation to configuration, reconfiguration, and the release of radio bearers (RBs). In this case, the RBs refer to a logical path provided by the first and second layers of the radio protocol for data transmission between the UE and the UTRAN. In general, the configuration (or establishment) of the RB refers to the process of stipulating the characteristics of a radio protocol layer and a channel required for providing a particular data service, and setting the respective detailed parameters and operational methods.
Hereinafter, the RLC layer will be explained in more detail. As aforementioned, the RLC layer operates in three modes, TM, UM, and AM. Since the RLC layer performs a simple function in the TM, only the UM and AM will be explained.
The UM RLC generates each PDU with a PDU header including a Sequence Number (SN), thereby allowing a receiving side to know which PDU has been lost while being transmitted. Accordingly, the UM RLC transmits broadcast/multicast data, or transmits real-time packet data such as voice (e.g., VoIP) of a Packet Service domain (PS domain) or streaming on a user plane. Also, on a control plane, the UM RLC transmits, to a specific terminal or specific terminal group in a cell, an RRC message requiring no response for reception acknowledgement.
Like the UM RLC, the AM RLC generates each PDU with a PDU header including a Sequence Number (SN). Differently from the UM RLC, in the AM RLC, a receiving side performs acknowledgement for PDUs transmitted from a sending side. In the AM RLC, a receiving side performs a response so that PDUs having not received can be re-transmitted by a sending side. The re-transmission function is the main characteristic of the AM RLC. An object of the AM RLC is to guarantee error-free data transmission using the re-transmission function. To this end, the AM RLC transmits non-real time packet data such as TCP/IP of PS domain in a User Plane, and transmits RRC messages requiring response of reception acknowledgement from a terminal in a cell in a Control Plane.
The UM RLC is used in a uni-directional communications system, whereas the AM RLC is used in a bi-directional communications system due to feedback from a receiving side. The UM RLC is different from the AM RLC in the aspect of structure. The UM RLC has one of a sending side and a receiving side in one RLC entity, whereas the AM RLC has both a sending side and a receiving side in one RLC entity.
The AM RLC is complicated due to a re-transmission function for data. The AM RLC is provided with a re-transmission buffer as well as a sending/receiving buffer. The AM RLC performs many functions, e.g., usage of a sending/receiving window for flow control, polling to request a Status Report from a receiving side of a peer RLC entity by a sending side, a receiving side's Status Report informing its buffer status to a sending side of a peer RLC entity, and Status PDUs to transmit status information.
In order to support the above functions, the AM RLC requires various protocol parameters, status variables, and timers.
In the AM RLC such as Status Report or Status PDUs, PDUs used to control data transfer is referred to as ‘Control PDUs’, and PDUs used to transfer User Data is referred to as ‘Data PDUs’.
Data loss may occur on a physical channel in all mobile telecommunications systems. A data loss rate, indicating that data was not successfully transmitted to a receiving side from a sending side on a physical layer is lower in an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) than in the related art systems. However, the data loss rate in the E-UTRAN is not completely ‘zero’. Especially, terminals located far from a base station have a high data loss rate. Accordingly, important signaling data or TCP data required to be transmitted without error have to be managed with more special care. To this end, the aforementioned Acknowledged Mode (AM) is used.
In the AM, when a receiving side has failed to receive data, the receiving side has to rapidly inform the status to a sending side. The less time taken for the receiving side to recognize errors and to inform the errors to the sending side is, the less time taken to correct the errors is. Also, time to transmit User Data is shortened, thereby enhancing a user's satisfaction degree. Accordingly, on an RLC layer, through an RLC Status Report, the receiving side informs its buffer status to the sending side, and requests re-transmission about data having not been received.
However, if the RLC receiving side transmits a Status Report whenever any missing data is found, radio resource is wasted. The RLC Status Report is transmitted through an RLC Status PDU. Since the RLC Status PDU is also one type of RLC PDU, the RLC Status Report to be transmitted requires radio resources, which it would waste radio resources in a mobile communications system.
Accordingly, required is a mechanism allowing an RLC Status Report to be rapidly transmitted, and consuming less radio resources.